Sometimes I'll grab something that interests me if the price is right. After that, assuming it's a part of a set (i.e. rear der., vs a saddle), I'll be on the lookout for complimentary components to complete the set/group even if I don't have any immediate plans for them. A set of deer head derailleurs is a recent example which are just about to be put into service. I've had the rear for a handful of years, snagged the front for a good price a couple years ago (passed on a matching set of thumbies at the same time - wish I hadn't though I might have never used them). I'm finally putting the derailleurs to use now. And there's the set of canti's for sale atm, but I'm trying to be good... I have a road and MTB pair of XC Pro rear mechs looking for their compatriots. etc...